The free radical polymerization of ethylenically unsaturated monomers is known. Polymers formed by this mechanism from monomers or oligomers having acrylic, methacrylic, vinyl ester and styrenic functionalities are major constituents in many films and cured layers, including protective layers, release layers, and adhesives. Polymerization typically involves the use of an added compound (an “initiator”) that initiates the reaction of and chain formation by such monomers. When free-radical initiators are used, the initiation step typically consists of two reactions. In the first reaction, the initiator undergoes cleavage or dissociation upon exposure to a source of radiation (e.g., heat, ultraviolet light, etc.), causing the formation of at least one radical species of the initiator. In a second step, this radical then combines with a first monomer to form a chain initiating species of the polymer. Once formed, this chain initiating radical propagates the polymerization reaction, incorporating other monomers into a growing polymer chain.
When electromagnetic radiation is the source of energy used to initiate and polymerize free radically reactive monomers, photoinitiators that absorb light and form radical species when exposed to energy in the ultraviolet to visible range (250 to 700 nm) are typically employed. These photoinitiators may be organic, organometallic, or inorganic compounds, but are most commonly organic in nature. Examples of commonly used organic free radical photoinitiators include benzoin and its derivatives, benzil ketals, acetophenone, acetophenone derivatives, benzophenone, and benzophenone derivatives. Alternatively, electron-beam (e-beam) radiation may be used without a photoinitiator to induce formation of radical species which can initiate chain growth and (co)polymer formation.